The present invention relates to silicon microflow devices, particularly to coating the inside surfaces of silicon microflow devices, and more particularly to a process for forming a conformal chemically resistant coating on deeply-recessed cavities with aspect ratios of up to 40:1 and higher.
Many different types of microflow devices and systems are being developed for wide ranging applications, particularly for analytical systems. One of the key limitations of microflow devices is the ability to make them resistant to chemical attack. The standard method for building microflow devices is to etch flow channels into the surface of a material (e.g., silicon wafer) and then bond wafers together to form the microflow system or device. If the material is not resistant to the environment for which the device is intended, the etched surfaces need to be coated with a chemically resistant material either before or after the bonding procedure. Coating the surfaces before bonding usually interferes with the bonding process (e.g., the coated surfaces usually do not adhere well to one another). Coating the surfaces after bonding represents a challenge in that most of the surfaces to be protected are buried inside of a microflow system with small and isolated access ports. Furthermore, thick coatings which can withstand chemical attack for extended periods tend to form stress-induced cracks which render such films useless.
Of particular interest is the need for a coating which is resistant to caustic (basic) solutions. Silicon and glass, the two most common materials to build microflow systems or devices, are already resistant to solvents and acids.
Corrosion protection of silicon from caustic solutions using a protective layer of silicon nitride has been previously shown. See J. G. E. Gardeniers, et al., xe2x80x9cCorrosion of Protected Layers on Strained Silicon Surfaces in Alkaline Solutions,xe2x80x9d Sensors and Materials; 5, 4, 189-208 (1994). While this paper showed the resistance of silicon nitride to caustic solutions, there remains a need for conformally coating silicon with silicon nitride, particularly in recessed cavities. Another prior approach has been to form silicon nitride channels in a silicon member and then bonding to channels to a glass substrate. See U.S. Pat. No. 5,824,204 issued Oct. 20, 1998 to J. H. Jerman.
It has also been established that simple deposition of 1-2 xcexcm thick silicon nitride films by conventional deposition techniques resulted in microcracks in the silicon nitride which are not visible by any standard microscopic techniques until the film is exposed to a caustic solution. Then the microcracks are xe2x80x9cdecoratedxe2x80x9d by the solution and the film degrades quickly because the underlying silicon is rapidly attacked by the caustic solution.
The present invention is directed to a process to coat the inside surfaces of silicon microflow devices with a low-stress, conformal (uniform) silicon nitride film which has the ability to penetrate deeply into recessed cavities (of aspect ratios up to 40:1 or higher) without microcracks in the film. The process involves low-pressure (vacuum) chemical vapor deposition. The 1-2 xcexcm thick silicon nitride film produced by the process of this invention enables extended exposure of silicon devices to caustic solutions. Thus, by this process, microflow systems fabricated in silicon will be resistant to all classes of chemicals: acids, bases, and solvents.
It is an object of the present invention to provide silicon microflow devices which are resistant to all classes of chemicals.
A further object of the invention is to provide a process for depositing silicon nitride films which are free of microcracks.
A further object of the invention is to provide conformal chemically resistant coatings for microflow devices or systems.
Another object of the invention is to provide a process for coating the inside surfaces of silicon microflow devices with a low-stress, crack-free, conformal silicon nitride film.
Another object of the invention is to provide a process which enables uniform coating of silicon nitride in deeply-recessed cavities with aspect ratios of up to 40:1 or higher.
Another object of the invention is to provide an ultra-low-stress silicon nitride deposition process which enables 1-2 xcexcm thick deposition without cracks, and so enables extended chemical protection of silicon microflow devices against caustics for up to 1 year.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawing. Basically, the present invention involves a conformal (uniform) chemically resistant coating for silicon microflow devices or systems. The invention involves a process for coating the inside surfaces of silicon microflow devices with a low-stress, crack-free, conformal silicon nitride film which has the ability to uniformly coat deeply-recessed cavities with aspect ratios of up to 40:1 or higher. The process is a low pressure (vacuum) chemical vapor deposition process which can be used at both ambient and elevated temperatures up to 65xc2x0 C., and can produce silicon nitride films with thicknesses of 1-2 xcexcm without cracks, such film thickness being needed to withstand caustic exposure for 1 year. By the ultra-low-stress process of this invention silicon nitride films can be uniformly deposited crack free on the inside silicon surfaces, whereby prior known chemical vapor deposition techniques were insufficient to achieve the thick (1-2 xcexcm) films required.